Problem: A circle has a circumference of ${18}$. It has an arc of length $6$. What is the central angle of the arc, in degrees?
Explanation: The ratio between the arc's central angle $\theta$ and $360^\circ$ is equal to the ratio between the arc length $s$ and the circle's circumference $c$. $\dfrac{{\theta}}{360^\circ} = \dfrac{{s}}{{c}}$ $\dfrac{{\theta}}{360^\circ} = {6} \div {18}$ $\dfrac{{\theta}}{360^\circ} = \dfrac{1}{3}$ ${\theta} = \dfrac{1}{3} \times 360^\circ$ ${\theta} = 120^\circ$ ${18}$ ${6}$ ${120^\circ}$